1. Field of the Art
The present invention relates to a process and an apparatus for producing an alloy containing terbium and/or gadolinium, and more particularly to such a process for continuously producing an alloy having a high content of terbium and/or gadolinium, and having a low control of both harmful impurities and non-metallic inclusions.
2. Related Art Statement
Terbium (Tb) and gadolinium (Gd) are utilized in the form of a thin-layered amorphous alloy of TbFe, TbCo, GdFe, GdCo, TbFeCo, TbGdFe, TbGdCo, etc. as a material for magnetooptical discs of rare earth type which have been recently studied and developed. These elements are also utilized for addition thereof to other kinds of material. The demand for terbium and gadolinium will be increased in the future. Although terbium or gadolinium in the form of a pure metal can be used to obtain an alloy containing the same, an alloy of terbium or gadolinium with iron, cobalt, or other alloying metal is preferable to handle for the addition thereof to other materials, since metallic terbium and metallic gadolinium have a comparatively high melting point, 1365.degree. C. for terbium and 1313.degree. C. for gadolinium.
Four processes of manufacturing an alloy of a rare earth metal with a metal of high melting point are described below, which are commonly known in the art. All of them, however, can not be satisfactory because of certain inherent disadvantages or problems, as the practical and industrial process operable continuously.
(A) One method requires a rare earth metal or its alloy to be prepared beforehand by means of electrowinning the same in a bath of electrolyte or by means of reducing a rare earth compound with an active metal; then the obtained rare earth or its alloy is melted together with another metal to alloy them:
This method, however, is problematical in the first step of preparing the rare earth or its alloy. In the electrowinning method, two techniques are known (1) electrolysis in an electrolyte bath of fused chrorides (raw materials), and (2) electrolysis of rare earth oxides (raw material) dissolved in an electrolyte bath of fused fluorides. The former technique suffers from the problem of difficulty associated handling of the fused chrorides, and the further problem resulting from the batch style processing which is not suitable for a continuous operation on a large scale. On the other hand, the latter technique has the problem of a low solubility of the oxide in the electrolyte bath, which hinders a continuous electrolysis operation and results in an accumulation of sludge on the bottom of the electrowinning cell. Therefore, for continuous and large scale production it is recommed that the rare earth or its alloy be produced in a liquid state, but it is impractical to raise to an excessively high electrolysis temperatures at which the electrolysis operation is conducted, according to a high melting point of the rare earth to be obtained, since at higher temperatures impurities and non-metallic inclusions more easily enter into the liquid rare earth or its alloy.
On the other hand, the reduction method utilizing an active metal belongs to a batch system and is, therefore, not suitable for continuous and large scale production. Further, this method has the disadvantage of requiring an expensive active metal (reducing agent) as well as expensive materials for the exclusive apparatus used in the method. This method has another disadvantage involving the additional step of removing the residual active agent.
(B) In another method alloying is executed by reducing a mixture of a rare earth compound and a metal compound to be alloyed with the rare earth through utilization a reducing agent (e.g., calcium hydride for a Sm-Co alloy).
This method requires an expensive reducing agent, and is unsuitable for a continuous and large scale operation.
(C) In another method an alloy of rare earth and a metal to be alloyed with the rare earth is electrodeposited on the cathode by electrolytic reduction, which reduction is carried out in a bath of electrolyte by dissolving both a compound of the rare earth and a compound of the metal to be alloyed with the rare earth (See U.S. Pat. No. 3298935.
This method is problematical in that it is difficult to keep the chemical composition of the alloy produced on the cathode uniform over a long period of time during the electrolysis operation. Further, in the case where an oxide is used as a raw material, a problem arises concerning low solubility of the oxide in the electrolyte bath, which hinders a continuous electrolysis operation.
(D) In the so-called consumable cathode method, rare earth is electrodeposited by electrolytic reduction on a consumable cathode of a metal and alloyed with the metal of the cathode, in one step which is executed in a suitable bath of electrolyte composed of fused salts See "U.S. Bur. of Min., Rep. of Invest.", No. 7146, 1968, and Japanese patents No. 837401 and 967389).
The shortcomings will be described hereinafter. In the case where a rare earth oxide is used as a raw material to be reduced, the method as stated previously suffers problems, of a low solubility of the rare earth oxide in the selected electrolyte bath and of an accumulated sludge of the oxide; moreover, conducting the electrolysis operation at increased temperatures in order to overcome those problems results in producing a deteriorated alloy containing an increased amount of impurities and non-metallic inclusions which impurities come from the structural materials of the electrowinning cell. Further, the recovery of the produced alloy is carried out in a batch style which is unsuitable for a continuous and large-scale operation.
Metallic terbium and metallic gadolinium have been, in fact, almost useless, and the industrial manufacturing process of obtaining the same has not been settled, except for the above-mentioned reduction method (A) in which terbium or gadolinium can be produced in a small quantity. However, the reduction method is not satisfactory in that the residual reducing agent (calcium) and the impurities (e.g., oxygen) are harmful to the "target" product, terbium or gadolinium. Therefore, it can be said that no industrially practical process is firmly established for continuously producing such metals.